Osmometer

ABSTRACT

An osmometer of the type having an osmotic membrane rigidly fitted to a cell having an inner space filled with a reference solvent is disclosed. The outer surface of the membrane is adapted to be in contact with the liquid solution whose osmotic pressure is to be measured. The inner surface of the membrane is in contact with the solvent. A pressure transducer membrane having two sides is positioned with one side in contact with the solvent so as to respond to the osmotic pressure developed across the osmotic membrane. Means are provided for transmitting to each side of the transducer membrane any change of mechanical and hydraulic pressure that may occur at the osmotic membrane whereby the transducer membrane responds only to the osmotic pressure. In one embodiment means are also provided for balancing the osmotic pressure with a hydraulic pressure.

lites States Patent Gilbert [l5] 3,635,7 [4's] Jan. 18, 1972 USMOMETER[72] Inventor: Paul '1. Gilbert, Los Altos Hills, Calif.

[73] Assignee: Beckman Instruments, Inc.

[22] Filed: June 29, 1970 [21] App]. No.: 50,460

[57] ABSTRACT An osmometer of the type having an osmotic membranerigidly fitted to a cell having an inner space filled with a referencesolvent is disclosed. The outer surface of the membrane is adapted to bein contact with the liquid solution whose osmotic pressure is to bemeasured. The inner surface of the membrane is in contact with thesolvent. A pressure m transducer membrane having two sides is positionedwith one [5 l Int. Cl. ..G0ln 11/00 Side in Contact with the solvent soas to respond to the osmotic [58] Field of Search ..73/64.3, 398, 401pressure developed across the osmotic membrane Means are provided fortransmitting to each side of the transducer mem- [56] References Citedbrane any change of mechanical and hydraulic pressure that UNITED STATESPATENTS may occur at the osmotic membrane whereby the transducermembrane responds only to the osmotic pressure. In one em- Ehrmantrautet al ..73/64-3 bodiment means are also provided for balancing thesmotic pressure with a hydraulic pressure. Primary Examiner-Donald O.Woodie] Attorney-William F. McDonald and Robert J. Steinmeyer 11 Claims,5 Drawing Figures SHEET 1 BF 2 INVENTOR- PAUL T. GILBERT ATTORNEYPATENIEDJAHBHR MI 2 BF 2 3,835,075

FIG. 5

ATTORNEY OSMOMETER BACKGROUND OF THE INVENTION The instant inventionrelates to'devices for measuring the osmotic pressure of a solution.

A particularly effective type of osmometer is that known as the Hansenosmometer and described by A. T. Hansen in Acta Physiol. Scand. 53, 197(1961). The Hansen osmometer requires a cell having an inner space ofsmall volume filled with a reference solvent. A circular osmoticmembrane of relatively large area is tightly held, clamped around itsedge, against the smooth, slightly convex surface of the cell body. Themembrane is exposed to direct contact with the inner space only over avery small central area. The rest of the inner surface of the membraneis in hydraulic contact with the inner space only indirectly through thethin film of solvent trapped between the membrane and the solidsupporting surface of the cell body, but pressure is transmitted throughthis thin film with little impedance. The outer surface of the membraneis placed in contact with the liquid solution whose osmotic pressure isto be measured. The solvent in the inner space is confined by theosmotic membrane, the walls of the cell, and the diaphragm or membraneof a pressure transducer responsive to the pressure of the solvent. Whenthe sample solution differs from the filling solvent in osmoticpressure, osmotic flow takes place through the osmotic membrane untilthe resulting change of pressure stops the flow. The pressure istransmitted through the thin film between the membrane and the cell bodyand through the inner space to the transducer membrane.

The Hansen osmometer is characterized by an extremely high elastance orstiffness of the system comprising the osmotic membrane, the solvent inthe inner space, and the membrane of the pressure transducer. That is, alarge pressure is required to cause a very small change in volume ofthis system. High elastance is imparted to the osmotic membrane byvirtue of its being rigidly supported against the smooth solid surfaceof the cell except for the very small unsupported area in contact withthe inner space. It therefore moves very little in response to areduction of pressure in the inner space. The solvent has high elastanceby virtue of the rigidity of the walls of the cell and the small volumeof the solvent, minimizing the effect of its compressibility. Thetransducer membrane is given high elastance by proper choice ofdiameter, thickness, and elastic modulus. It is this combination ofextremely high elastance of the system, usually of the order of [Lb/Pl(microbars per picoliter), and large area of the osmotic membrane, ofthe order of 1 cm that enables the osmotic pressure to attainequilibrium very quickly. Hansen achieved a time constant of 5 seconds,the fastest heretofore reported for a membrane osmometer.

An important disadvantage of the Hansen type osmometer, which isunrelated to the nature of the pressure transducer, is that thetransducer measures the net or gauge pressure of the solvent within thecell, and this measurement includes the hydrostatic pressure, i.e.,hydraulic or mechanical pressure exerted by the sample upon themembrane. Because of the quickness of response of the system, any changein this pressure causes a flow of the solvent through the membrane thatrapidly balances it. The membrane transmits mechanical and hydraulicpressure freely. Thus, the measured osmotic pressure is afflicted witherrors due to hydrostatic pressure if, for example, the depth of aflowing sample varies or if the depths of sample and a test blank arenot equal.

When high sensitivity is wanted, it can be quite hard to control thehydrostatic pressure of a sample with the requisite precision. If thetransducer measures absolute rather than gauge pressure, it alsoresponds to changes of atmospheric pressure. Barometric fluctuations canspoil the precision in the micromolar range of concentration. If thetransducer measures gauge pressure in the cell, it still detectsinhomogeneities of atmospheric pressure in the small region between thesurface of the sample and the side of the transducer element exposed tothe atmosphere. Over a distance of a few centimeters, this inhomogeneitycan cause fluctuations of approximately one microbar unless theinstrument is carefully shielded from breezes. Such fluctuations can beminimized by having the transducer as close as possible to the sample.

Fluctuations of temperature, although less detrimental, can cause someproblems. As the cell warms up, the filling solvent expands, creatingpressure. The expansion also changes the density of the solvent andtherefore slightly changes the hydrostatic head of the solvent insidethe cell. The change of internal mechanical pressure generated by thesethermal effects is promptly relieved by flow through the osmoticmembrane in the same way as an external change of hydrostatic pressureor head. Such balancing of pressure due to changes of temperature orhead takes place whether or not an osmotic pressure is beingexperienced. The mechanical or hydraulic pressure behaves independentlyof the osmotic pressure.

There is another difficulty resulting from the nature of certain typesof highelastance pressure transducers advantageously used in anosmometer of the Hansen type. Such pressure transducers have very smallmembranes whose motion is most conveniently observed and measured withan optical lever, i.e., a beam of light reflected from the surface ofthe membrane itself to a photosensitive detector. Transducers of thiskind, especially those possessing the highest elastance (approaching I00ub/pl) together with the highest sensitivity (approaching 0.1 ab), givea linear response, i.e., a signal proportional to the pressure, overonly a limited range of pres sure. Such transducers in which themembrane is liquid are also physically incapable of sustaining a largerange of pressure. The concentration of a solution having an osmoticpressure corresponding to the greatest linear response of such apressure transducer may be no more than 10 micromoles per liter, whilethe concentration corresponding to the greatest sustainable pressure maybe of the order of l millimole per liter.

SUMMARY OF THE INVENTION It is an object of the instant invention toprovide an osmometer of the type described wherein changes of mechanicaland hydraulic pressure are compensated so that their influence upon theosmotic pressure measured is eliminated. Advantageously, this isaccomplished without adversely affecting the response time andsensitivity of the instrument.

It is another object of the invention to provide an osmometer whereinthe osmotic pressure is balanced by an externally applied hydraulicpressure that can be measured. The burden of measurement is therebyremoved from the pressure transducer, which serves only to indicate thepoint of balance. Limitations of linearity and sustainable pressure inthe transducer are thereby also removed. It becomes possible to combinevery high sensitivity with linearity over a substantially unlimitedrange of osmotic pressure.

The osmometer according to the instant invention has an osmotic membranerigidly fitted to a cell which has an inner space filled with areference solvent. The outer surface of the membrane is adapted to be incontact with the liquid solution whose osmotic pressure is to bemeasured. The inner surface of the membrane is in contact with thesolvent. A pressure transducer membrane having two sides is positionedwith one side in contact with the solvent so as to respond to theosmotic pressure developed across the osmotic membrane. Means areprovided for transmitting to each side of the pressure transducermembrane any change of mechanical and hydraulic pressure that may occurat the osmotic membrane, whereby the transducer responds only to theosmotic pressure. In one embodiment means are also provided forbalancing the osmotic pressure with a hydraulic pressure.

This description of the invention and the advantages thereof will becomeclearer from the following detailed description taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a front elevational view, insection, of one embodiment of the instant invention.

FIG. 2 is a partial front elevational view, in section, of anotherembodiment of the instant invention.

FIG. 3 is a partial front elevational view, in section, of an alternateembodiment of the instant invention.

FIG. 4 is a partial front elevational view, in section, of an alternateembodiment of the instant invention.

FIG. 5 is a front elevational view, in section, of another embodiment ofthe instant invention.

Throughout the drawings, the same reference numerals have been appliedto various corresponding parts.

DETAILED DESCRIPTION FIG. I shows an osmometer, indicated generally at10, including a cell 12 having an inner space 14 filled with a referencesolvent 16. A suitable osmotic membrane 18 is rigidly fitted to cell 12as by cap 20 and O-ring 22. As shown, membrane 18 is disposed over thegenerally spherical surface 24 of cell 12 in such a way that solvent 16is in contact with the entire inner surface 26 of the membrane. Theother side 28 of osmotic membrane 18 is in contact with the liquidsolution 30 whose osmotic pressure is to be measured. Solvent 16 is incontact with a pressure transducer membrane 32 which is shown sealedagainst space 14. This membrane may be either solid or liquid, but isshown as a liquid membrane of the type described in copendingapplication Ser. No. 50,462 filed June 29, 1970. In this example,membrane 32 is simply the terminal meniscus of solvent 16 at the lowerend of inner space 14. Inner space 14 desirably has a very smalldiameter, for example, about 0.1 millimeters, to impart a suitableelastance to transducer membrane 32. Lower face 34 of cell 12 isenclosed with a thin transparent window 36 of suitable material such asglass. Window 36 confines an air space 38 that is exposed only totransducer membrane 32, and, as will be shown hereinafter, to the samplesolution 30. The motion of membrane 32 under the influence of pressureis observed and measured, in this example, by means of an optical lever.The optical lever includes a light source 40 arranged so as toilluminate transducer membrane 32. Transducer membrane 32 is deflectedin response to the osmotic pressure in solution 30 either inwardly oroutwardly. The arrangement is such that membrane 32 will reflect thelight from light source 40 in a manner dependent upon the pressure beingsensed to a sensor or detector circuit such as is described in detail incopending application Ser. No. 50,463 filed June 29, I970. Morespecifically, the light from light source 40 which is focused uponmembrane 32 by lens system 42 is reflected in accordance with theosmotic pressure sensed to a photodetector 44. If desired, window 36 mayhave an antireflection coating to cut down stray light in the opticallever system.

Inner space 38 is airtight except for two passages 46, and 48. Passage48 includes a flexible hose 50 connected to a tube 52 that communicatesthrough cap 20 with solution 30. Solution 30 is thus free to flowthrough tube 52, hose 50, and passage 48 toward space 38. Passage 46 isconnected to a three-way stopcock 54, one of whose outlets 56 goes tothe atmosphere, and the other outlet 58 to a rubber bulb 60, Bymanipulation of stopcock 54 and bulb 60, solution 30 can be drawn downinto passage 48 until its meniscus 62 is in a portion of passage 48which is substantially horizontally disposed. Passage 48 desirably isnarrow enough that meniscus 62 will remain intact and solution 30 willnot spill down into space 38. With the stopcock 54 closed, meniscus 62will remain stationary or will move only slightly in response to changesof temperature in space 38 or to variations in the hydrostatic pressurein solution 30. These various pressures will be referred to asmechanical and hydraulic pressures. Any such motion will not alter theeffective elevation of meniscus 62 since the portion of passage 48 inwhich it is contained is substantially horizontal. Alternatively,meniscus 62 could be confined to tube 52, which is also substantiallyhorizontally disposed, with passage 48 and hose 50 being filled withair. The transducer membrane 32 is thus in hydraulic contact with thesolution 30 through airspace 38. Any changes in hydrostatic pressure inthe sample due to changes of depth or density and any changes inatmospheric pressure, acting on the sample, are transmitted throughpassages 52, 50, and 48 to the outer surface of transducer membrane 32,and simultaneously through osmotic membrane 18 and inner space 14 to theinner surface of transducer membrane 32. Such changes of hydrostatic oratmospheric pressure therefore have no effect upon membrane 32, Althoughthe two sides of membrane 32 may not be subjected to the same hydraulicand mechanical pressure, any difference of such pressure across themembrane remains constant. The transducer membrane 32 thus responds onlyto the osmotic pressure developed across osmotic membrane 18 inconsequence of differences of composition between the liquids on the twosides of membrane 18. The osmometer is thereby compensated formechanical and hydraulic pressure.

If tube 52 is sufiiciently small, the time constant of the compensatingsystem will match that of the osmometer. Thus, any rapid change orfluctuation of mechanical and hydraulic pressure acts with equal speedthrough the osmotic membrane 18 and the compensating system comprisingtube 52, hose 50, and passage 48, causing no momentary unbalance.Further, if the inner opening of tube 52 is small enough, the contentsof the tube will scarcely mix with the sample solution 30. Samples canthen be changed freely without trouble from contamination by thecontents of tube 52. In other words, tube 52, hose 50, and passage 48will not have to be flushed out every time samples are changed. It isdesirable to leave the same liquid, suitably solvent 16, in tube 52,hose 50, and passage 48 (unless passage 48 and hose 50 are filled withair) so that the liquid in the compensating system always exerts thesame mechanical and hydraulic head. Any difference of level between theinner opening of tube 52 and osmotic membrane 18 causes errors ofnegligible magnitude, due to differences of density between samples.

In the embodiment shown in FIG. 2, a liquid metal such as mercury 64 isused for forming transducer membrane 32. Mercury 64 is in a sphericalopening or hollow 78 in a cylinder 68, having an orifice 66, in whichtransducer membrane 32 is formed, at its lower end and an opening 82 atits upper end. Cylinder 68 fits in inner space 14, so that mercury 64 issurrounded by solvent 16. Thus, mercury 64 is entirely within cell 12. Aring clamp 70 and O-ring gasket 72 hold a transparent window 36 againstcell body 12 and supports cylinder 68 and transducer membrane 32. Ifwindow 36 and lower face 34 of cell 12 are optically flat, they may sealsimply due to the pressure. Alternately, a suitable cement can be usedto make the interface between window 36 and lower face 34 leaktight. Theupper surface 74 of cylinder 68 is slightly convex to match and continuethe curvature of spherical surface 24 of cell 12.

The lower surface of cylinder 68 has a conical aperture 76 communicatingwith mercury 64 in spherical hollow 78. The orifice 66 between hollow 78and conical aperture 76 accommodates transducer membrane 32, which maybe for example 0.2 mm. in diameter. Its design must meet therequirements discussed in copending application Ser. No. 50,462 filedJune 20, 1970. Motion of membrane 32 may be observed and measured bymeans of an optical lever like that of FIG. 1. The incident andreflected beams pass through window 36 and aperture 76. Orifice 82 atwhich the mercury 64 is in contact with osmotic membrane 18 is largerthan orifice 66, for example 2 mm. in diameter. The edges of orifice 82are desirably quite sharp and thin so that the surface of mercury 94nowhere has a radius of curvature smaller than about 1 mm. except atorifice 66 in response to osmotic pressure. Solvent 16 fills all of thespace inside cell 12 not occupied by other parts, for example, the thinspace between osmotic membrane 18 and cell 12, inner space 14 andconical aperture 76. The solvent in inner space 14 is in communicationwith the solvent in conical aperture 76. This can be accomplished simplyby having the bottom surface of cylinder 68 roughened so that it doesnot seat tightly on window 36 or by providing grooves in the lowersurface of cylinder 68. Solvent 16 must also be present between osmoticmembrane 18 and the top of the mercury exposed in orifice 82, so thatosmotic flow may take place through this part of the membrane 18,generating an osmotic pressure that will be transmitted to the mercury.

This device is compensated against mechanical and hydraulic pressurebecause the transducer membrane 32 is complete ly enclosed within thespace occupied by solvent 16 and hence is within the space characterizedby the high elastance of the overall osmometer 10. The two sides of thetransducer membrane 32 are in hydraulic contact with each other viasolvent 16. Any change of hydraulic or atmospheric pressure is rapidlytransmitted through osmotic membrane 18 and then acts uniformlythroughout the interior of inner space 14 of cell 12, including bothsides of transducer membrane 32.

The diameter of orifice 66 governs the elastance of transducer membrane32. The construction of the rest of osmometer 10, especially window 36,is desirably sufficiently rigid that the overall elastance of cell 12 isnot much less than that of transducer membrane 32 itself. Inner space 14is sufficiently narrow that sagging of osmotic membrane 18 into it willnot cause much loss of elastance. The soft" part of the cell envelope,i.e., the part of osmotic membrane 18 above orifice 82, transmits theosmotic pressure to transducer membrane 32. Here, the stiffness ofosmotic membrane 18 must be small compared to that of transducermembrane 32, so that the undesirable elastic properties of osmoticmembrane 18, such as nonlinearity, hysteresis, and drift, will notimpair the performance of transducer membrane 32.

It may be shown that:

where E, is the elastance of transducer membrane 32 and E that of theunsupported part of osmotic membrane 13 above mercury 64. 0' is thePoissons ratio, Y, the Youngs modulus, and 2,, the thickness of theosmotic membrane. T is the surface tension of mercury 64 against solvent16 and r is the radius of orifice 82 and r, the radius of orifice 66. I,is approximately dy/cmF, and Z is typically 0.01 cm. Thus, when r is atleast 0.1 cm., the ratio EJE, is high enough that transducer membrane 32dominates the elastance.

In the embodiment shown in FIG. 3, a solid transducer membrane 32 isused that will directly support osmotic membrane 13 on one side and isviewed from the other side by the optical lever system.

In FIG 3 the osmotic membrane 18 is clamped to cell 12 by cap 20 andO-ring 22. A strong transparent window 36 is clamped to lower face 34 ofcell 12 by ring clamp 70 and O- ring gasket 72. Inner space 14 is in theform of a truncated cone. The narrow end 84 of inner space 14 isadjacent osmotic membrane 18 and is closed by a transducer membrane 32of a highly elastic solid, e.g., silica. The motion of transducermembrane 32 will be observed by a beam from a light source (not shown inFIG. 3) which passes through window 36 and inner space 14 and isreflected from membrane 32 to the photocells (not shown) of the opticallever system. If desired, transducer membrane 32 may be lightly silveredfor better reflectance.

Solvent 16 completely fills inner space 14 and the thin space betweeninner surface 26 of osmotic membrane 18 and spherical surface 24 of cell12. A passage 36 provides hydraulic communication between this space andinner space 14 to equalize the pressure on the two sides of transducermembrane 32. The osmometer 10 is desirably rigid and leaktight so thatits overall elastance will be essentially that of transducer membrane32. A film of solvent 16 desirably is present between osmotic membrane18 and transducer membrane 32 as in the osmometer shown in FIG. 2. Anychange of hydrostatic or atmospheric pressure, i.e., any change inmechanical and hydraulic pressure, is equalized across osmotic membrane18 by virtue of its permeability and across transducer membrane 32 byflow through passage 36. Osmotic pressure of a sample solution 30 causesoutward flow of solvent 16 through osmotic membrane 18. Membrane 18 thenpresses against the entire spherical surface 24 of cell 12, includingtransducer membrane 32 which thus registers the osmotic pressure.

It will be appreciated that in all cases osmotic membrane 18 readilytransmits mechanical and hydraulic pressure which equilibrates with thesame speed as osmotic pressure. Consequently, it does not matter whetherthe two sides of transducer membrane 32 are in hydraulic contact withliquid on the same side of osmotic membrane 18, as in the embodiments ofFIGS. 2 and 3, or on opposite sides of it, as in FIG. 1.

The embodiment shown in FIG. 4 may be regarded as a simplified variationof that shown in FIG. 3. The separate solid transducer membrane shown inthe arrangement of FIG. 3 has been eliminated. The small unsupportedarea of osmotic membrane 18 that covers inner space 14 now serves astransducer membrane 32. In other words, the transducer membrane issimply a part of the osmotic membrane. In the example shown, inner space14 is merely a small recess filled with solvent, lying in the uppersurface 24 of cell 12. In order to observe and measure the deflection ofunsupported membrane area 32, an optical lever can be placed above thecell, in the position shown. A light source 40 is focused by lenses 42upon membrane area 32, which reflects the beam to photodetector 44. Inthis case the incident and reflected beams must pass through the samplesolution 30 which is placed in contact with osmotic membrane 18. Toavoid distortions of the beam due to disturbances of the surface ofsolution 30, the solution is confined above by a window 112 mounted oncap 20. Channels 114 in cap 20 are provided to permit injecting andremoving solution 30 into and from the space confined by membrane 18,cap 20, and window 112. Alternatively, the arrangement of FIG. 3 couldbe employed, in which conical inner space 14 is confined above bymembrane area 32 (which is part of osmotic membrane 13) and below bywindow 36. The beam of the optical lever would in that case pass throughwindow 36 and inner space 14 rather than through window 112 and solution30. The embodiment of FIG. 4 functions by virtue of the elasticity ofunsupported membrane area 32, which bulges downward in response to theosmotic pressure that arises when the concentration of solution 30 isgreater than that of reference solvent 16 in inner space 14. Membranearea 32 does not move in response to changes of mechanical and hydraulicpressure, which it freely transmits. It responds to osmotic pressureonly. With a thickness of typically 0.1 mm., an unsupported area of atypical osmotic membrane will exhibit an elastance of IO to I00 zb/plwhen its diameter is about 1 mm. or a little less, a very convenientvalue.

The embodiment of the instant invention shown in FIG. 5 can be lookedupon as a variation of that shown in FIG. 1. It provides compensationfor changes of mechanical and hydraulic pressure in the same way, and itadditionally provides means for balancing the osmotic pressure with anexternally applied hydraulic pressure that can be measured, therebyconverting the osmometer into a null-balance instrument and greatlyextending its range of linear measurement without sacrifice ofsensitivity. In FIG. 5, as in FIG. 1, transducer membrane 32 desirablyis a liquid membrane formed by the terminal meniscus of solvent 16 atthe lower end of inner space 14. A variation is shown in that lower face34 of cell 12 is machined flat but at a slight angle, not parallel totransducer membrane 32. Window 36 is then clamped against lower face 34by ring clamp 70 and O-ring gasket 72. Being tilted at an angle, window36 will not reflect light from the light source to the photodetector. Ifdesired, window 36 may be given an antireflection coating to minimizestray light.

Inner space 38 is connected to hollow passage 43 extending through cell12 with a horizontal portion substantially at the same level as thetransducer membrane 32. Passage 48 then extends upwardly and has asecond horizontal portion 88 at the same level as osmotic membrane 18.Second horizontal portion 88 desirably has a capillary constriction toinhibit diffusion or convection of liquid when density differences existbetween the two ends of horizontal portion 88. Second horizontal portion88 then forms a T-joint with a vertical portion 90 of passage 48. Theupper end 92 of vertical portion 90 bends around and enters solution 30with its open end 94 extending below the surface thereof. Open end 94desirably is placed low in solution 30. A valve 96 is provided in upperend 92 of vertical portion 90 of passage 48 so as to separate thepassage into two sections, a sample section indicated generally at 98and a transducer membrane section indicated generally at 100. Valve 96is desirably so designed that when opened and closed it displaces noliquid in passage 48. The lower end 102 of vertical portion 90 ofpassage 48 enters the bottom of a vertical cylindrical reservoir 104which has a drain valve 106 at its bottom. The top of reservoir 104 isdesirably at the same level as the top of osmometer 10. One or morecylindrical plungers, 108, 110 are supported vertically in reservoir 104and may be moved by suitable mechanisms, not shown, that also indicatethe height of plungers 108 and 110.

Reservoir 104 and vertical portion 90 of passage 48 are filled withsolvent 16. If solvent 16 is water, it may sometimes be desirable to adda suitable surface-active agent to reservoir 104 to insure reproduciblewetting of plungers 108 and 110 and smooth motion of meniscus 62. Withsolution 30 initially consisting of solvent 16, valve 96 and drain 106are opened to flush air from vertical portion 90, additional solventbeing added to maintain the desired level of solution 30. With drain 106closed and valve 96 open, the liquid level in solution 30 and reservoir104 will be equal. If desired, clamp 70 may be loosened momentarily toremove air from airspace 38 to bring meniscus 62 into the desiredposition in the first horizontal portion of passage 48, such that itwill not move out of that portion of passage 48 during subsequentmanipulations.

When valve 96 is open, the arrangement functions in substantially thesame manner as that of FIG. 1 to subject each side of transducermembrane 32 to the same mechanical and hydraulic pressure that prevailsat osmotic membrane 18 plus a pressure equal to the head of solvent ininner space 14, so that transducer membrane 32 will respond only to theosmotic pressure. Since second horizontal portion 88 is at the level ofosmotic membrane 18, a change of density of the liquid in verticalportion 90 when a sample solution 30 other than solvent is applied tothe osmometer will cause no unbalance of the mechanical and hydraulicpressure across transducer membrane 32. Solvent 16 is trapped betweenthe capillary in second horizontal portion 88 and airspace 38. Thissolvent 16 will exert the same head as the solvent in inner space 14.Hence, the pressure on both sides of transducer membrane 32 equals thehead of solution 30 plus the head of solvent 16 in inner space 14 or inpassage 48 up to the capillary in second horizontal portion 88.

If solution 30 has an osmotic pressure, solvent 16 will develop anegative pressure equal to the osmotic pressure drawing liquidtransducer membrane 32 upward. This negative pressure is then balancedafter closing valve 96 by changing the hydrostatic pressure intransducer membrane section 100. This is accomplished by raising one ofthe plungers 108, 110 in reservoir 104 so as to restore the balance ofpressure at transducer membrane 32. Any unbalance is indicated by theoptical lever system (not shown). The height reading on the plunger 108or 110 which has been moved to restore the optical lever system tobalanced position is a measure of the osmotic pressure. If, for example,plunger 108 has l/ll the cross section of the reservoir, a a verticalmotion of cm. will cause the water level to change by 1 cm.,corresponding to a pressure of about 1 millibar. Plunger 110 might havel/ 10 the cross section of plunger 108 to provide a ten fold greaterscale expansion. The choice of plungers depends on the expected osmoticpressure.

While there have been described hereinabove certain embodiments of thisinvention, it is to be understood that the invention is not limitedthereto and various changes, alterations and modifications can be madethereto without departing' from the spirit and scope thereof. Forexample, the pressure transducer membrane 32 in FIGS. 1 and 5 could be aconvcntional solid membrane and airspace 38 could be filled with liquid.lnstead of the system of plungers in a reservoir, a reservoir ofadjustable height with a flexible hose connection could be used or achamber enclosed by a diaphragm moved by a screw and having an attachedmanometer, or a micrometer syringe with a buffer gas space to serve as aspring could be used. The pressure developed by any such means could bedirectly indicated by any kind of pressure transducer.

Accordingly, the instant invention is not to be limited to theparticular embodiments disclosed, but only by the claims, wherein whatis claimed is:

1. ln an osmometer of the type having an osmotic membrane rigidly fittedto a cell having an inner space filled with a reference solvent, theouter surface of the membrane being adapted to be in contact with theliquid solution whose osmotic pressure is to be measured and the innersurface of the membrane being in contact with the solvent, and apressure transducer membrane having two sides and positioned with oneside in contact with the solvent so as to respond to the osmoticpressure developed across the osmotic membrane, the improvement whichcomprises means for transmitting to each side of the transducer membraneany change of mechanical and hydraulic pressure that may occur at theosmotic membrane whereby the transducer membrane responds only to theosmotic pressure.

2. The osmometer of claim 1 including a closed space surrounding thesecond side of the transducer membrane and containing a gas and whereinthe means comprise a hollow passage connecting the gas filled space andthe sample solution whereby the mechanical and hydraulic pressure in thesolution at the osmotic membrane is transmitted to the second side ofthe transducer membrane.

3. The osmometer of claim 1 wherein both sides of the transducermembrane lie within the inner space of the osmometer, and the innerspace includes a passage through which hydraulic pressure is transmittedfrom one side of the transducer membrane to the other, and one side ofthe transducer membrane is subjected to the osmotic pressure to bemeasured.

4. The osmometer of claim 3 wherein the transducer membrane is a liquidmembrane.

5. The osmometer of claim 3 wherein the transducer membrane is a solidmembrane.

6. The osmometer of claim 2 including means for controlling theinterface between the solution and the gas so that its motion will notresult in a change of mechanical and hydraulic pressure.

7. The osmometer of claim 6 wherein the hollow passage has asubstantially horizontal portion and the control means are adapted tomaintain the interface in the horizontal portion.

8. The osmometer of claim 2 including the following additionalcomponents:

a. a valve in the passage for separating the passage into two sections,a sample section and a transducer membrane section;

b. means for changing the mechanical and hydraulic pressure in thetransducer membrane section when the valve is closed.

9. The osmometer of claim 8 including means for measuring the change inmechanical and hydraulic pressure in the transducer membrane sectionwhich balances the osmotic pressure.

10. The osmometer of claim 8 wherein the pressure changing meansincludes a liquid containing reservoir connected hydraulically to thetransducer membrane section and a plunger movable in the liquid in thereservoir to change the mechanical and hydraulic pressure in thetransducer membrane section.

11. The osmometer of claim 1 wherein the transducer membrane is a partof the osmotic membrane.

1. In an osmometer of the type having an osmotic membrane rigidly fitted to a cell having an inner space filled with a reference solvent, the outer surface of the membrane being adapted to be in contact with the liquid solution whose osmotic pressure is to be measured and the inner surface of the membrane being in contact with the solvent, and a pressure transducer membrane having two sides and positioned with one side in contact with the solvent so as to respond to the osmotic pressure developed across the osmotic membrane, the improvement which comprises means for transmitting to each side of the transducer membrane any change of mechanical and hydraulic pressure that may occur at the osmotic membrane whereby the transducer membrane responds only to the osmotic pressure.
 2. The osmometer of claim 1 including a closed space surrounding the second side of the transducer membrane and containing a gas and wherein the means comprise a hollow passage connecting the gas filled space and the sample solution whereby the mechanical and hydraulic pressure in the solution at the osmotic membrane is transmitted to the second side of the transducer membrane.
 3. The osmometer of claim 1 wherein both sides of the transducer membrane lie within the inner space of the osmometer, and the inner space includes a passage through which hydraulic pressure is transmitted from one side of the transducer membrane to the other, and one side of the transducer membrane is subjected to the osmotic pressure to be measured.
 4. The osmometer of claim 3 wherein the transducer membrane is a liquid membrane.
 5. The osmometer of claim 3 wherein the transducer membrane is a solid membrane.
 6. The osmometer of claim 2 including means for controlling the interface between the solution and the gas so that its motion will not result in a change of mechanical and hydraulic pressure.
 7. The osmometer of claim 6 wherein the hollow passage has a substantially horizontal portion and the control means are adapted to maintain the interface in the horizontal portion.
 8. The osmometer of claim 2 including the following additional components: a. a valve in the passage for separating the passage into two sections, a sample section and a transducer membrane section; b. means for changing the mechanical and hydraulic pressure in the transducer membrane section when the valve is closed.
 9. The osmometer of claim 8 including means for measuring the change in mechanical and hydraulic pressure in the transducer membrane section which balances the osmotic pressure.
 10. The osmometer of claim 8 wherein the pressure changing means includes a liquid containing reservoir connected hydraulically to the transducer membrane section and a plunger movable in the liquid in the reservoir to change the mechanical and hydraulic pressure in the transducer membrane section.
 11. The osmometer of claim 1 wherein the transducer membrane is a part of the osmotic membrane. 